This undated file image posted on a militant website on Jan. 14, 2014, shows fighters from the al Qaida-linked Islamic State of Iraq and the Levant (ISIL) marching in Raqqa, Syria.

Militant website/AP Photo

Syria has risen as a top threat to the U.S. homeland that rivals Yemen's al Qaeda affiliate known for its innovative bombs successfully smuggled aboard airplanes, a top Pentagon official said on Thursday.

"Syria is probably the number one threat -- or, with... threats out of Yemen -- to the American homeland right now and elsewhere in the west," said Michael Vickers, the Pentagon’s Under Secretary for Defense Intelligence.

In answer to a question by moderator Brian Ross of ABC News at a panel at the Aspen Security Forum, an annual gathering of national security veterans, Vickers, himself a former Special Forces operator, said that the foreigners flocking to Syria are hard to track and are a "serious problem."

"Foreign fighters who are Western passport holders, including Americans, a subset of that, numbers in the four digits," Vickers said.

PHOTO: Seen in this image is "Abu Dujana al-Amriki," who identifies himself as an alleged al Qaeda fighter from the U.S., but who U.S. officials believe to be part of an Assad regime hoax.

Since 2010, al Qaeda in the Arabian Peninsula in Yemen has held the top spot in the minds of counter-terrorism officials after the terror group tried unsuccessfully to use four different improvised explosive devices, hidden in underwear on a bomber or inside printer cartridges, in an effort to blow up U.S.-bound commercial passenger and cargo jets.

But a portion of the thousands of foreign fighters joining extremists for training and jihad against Syria's embattled dictator Bashar al Assad over the last few years are viewed as likely to eventually try to attack western targets, many officials have said.

Top U.S. officials have spoken more openly about the threat emanating from Syria this year.

John Carlin, the assistant attorney general for national security at the Justice Department, has established a prosecutors unit to focus on Americans fighting in Syria or aspiring to, because of the enormity of the threat.

"The number of foreign fighters that are already in place in Syria, and a number of other westerners in that group, is one that is unprecedented, and is a larger number than we ever saw in ungoverned spaces in Pakistan and Afghanistan," Carlin said during the panel discussion.

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PHOTO: A rebel Islamist group in Syria posted this image online, claiming it showed an American that had taken part in a suicide operation against Syrian government forces.

Homeland Security Secretary Jeh Johnson said it's all his European colleagues want to discuss, their fears are so pronounced. "It's number one on the list of discussion topics," he told the security conference.